Caloric restriction triggers a number of homeostatic endocrine responses including suppression of leptin, gonadotropins, sex steroids, thyroid hormones, insulin and insulin like growth factors (IGFs) and elevation of growth hormone (GH) and cortisol. Some of these effects may be disadvantageous in terms of achieving loss of adipose tissue. There is little information on the role of specific macronutrients in the regulation of these important endocrine responses to food restriction. The present study aims to determine if dietary carbohydrate and/or lipid composition has significant effects on these hormonal regulators of body composition and metabolism, independent of total calorie supply. This will be achieved by comparing the short-term effects of isocaloric, weight maintenance diets restricted in either fat or carbohydrate with a standard balanced diet. Protein contribution as well as calorie content will be held constant in the 3 diets. Study subjects will be obese women and men (BMI 30-34.9 kg/m2) aged 18-40 years. Major study outcome parameters include: leptin, gonadotropin, GH and cortisol secretory dynamics; sex steroid, IGF, fasting insulin and glucose levels.